Non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries provide high voltage and have high energy density and capacity, and they are expected as promising power sources of various devices. Particularly for those used as driving power sources of mobile devices, hybrid electric vehicles, electric vehicles, and the like, a higher capacity is required to further improve the operating time and the driving distance.
Under these circumstances, examinations have been made to increase the energy density of non-aqueous electrolyte secondary batteries.
For example, Patent Literature 1 discloses that, in a non-aqueous electrolyte secondary battery having a volume energy density of about 300 Wh/L, the volume of liquid non-aqueous electrolyte contained therein be controlled to 120 to 140% of the total pore volume of the positive electrode, negative electrode, and separator, in view of the cycle characteristics and safety. Here, the volume energy density is an energy density of a battery per volume specified by the size of a case of the battery.
Patent Literature 2 combines ethylene carbonate (EC) or propylene carbonate (PC) with ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC), as a non-aqueous solvent to be included in a non-aqueous electrolyte for a lithium ion secondary battery. Patent Literature 2 discloses that by combining solvents as above, the viscosity of the non-aqueous electrolyte is reduced to improve the permeability thereof into the separator, and thus a sufficient discharge capacity can be obtained.